Full-color printing on offset presses has become relatively reliable and affordable for clients long accustomed to printing in black and white or with just one or two pre-mixed spot inks. Such printing utilizes photo-chemical processes to reduce original multi-colored materials to the four constituent colors used in printing. For example, printed color images combine different intensities of four basic colors—Magenta (“M”), Yellow (“Y”), Cyan (“C”), and Black (“K”)—using a printing process known as four-color-process printing. In practice, accurately printing a color image to a customer's satisfaction is often times tedious, problematic and time consuming, as it usually requires manual intervention. For example, conventional four-color-process printing usually utilizes presses that are only designed to either apply or not apply a single amount of ink to any given location on a page. To reduce the number of errors and expenses associated with errors in acceptable print quality off the press, proofs are usually used.
Four-color-process printing requires a reliable color proof for use as a guide for press operators and customers in finalizing a printing press to perform a production print job. For example, the proof conveniently and inexpensively provides one set of values for each of the print colors to be used on the production print job, and an easily-changed and viewable image for both the press operator and the customer. A single piece of film for each of the four colors is also required by platemaker to make thin printing plates that are wrapped on the drums of the printing press, covered with the appropriate inks, and then offsets from blankets are rolled over sheets of paper during the printing process. Computer-to-Plate (CTP) technology can eliminate the need for film in the plate-creation process. Unfortunately, a proof includes inherent tone and color differences from a press sheet, and a great deal of time is consumed in assessing how to improve the coincidence of the tone and color reproduction characteristics of a proofing system to those of a press.
Specifications for Web Offset Publication (“SWOP”) provide the official set of standards for the publication printing industry and also have become a defacto standard used by the remainder of printing industry. Among other things, SWOP specifies the density or degree of light absorption, in an area that prints solid for the C, M, Y, K proofing colorants and printing inks (collectively “the colorants”) and also specifies a tonal appearance weight that should appear in an area that prints 50% screened. This tonal appearance weight is impacted not only by a printing device's reproduction characteristics, but also by density values of printed solid areas. This density value is typically varied by varying an ink-film thickness.
The SWOP specification for a 50% screened area is stated in terms of dot gain, which represents a difference in dot area between an input film printing dot area and the apparent dot area measured on a printed sheet. The computed value includes both physical changes in dot size and optical effects that increase the apparent size of the printed dot. For example, high dot gain value is intended to indicate higher tonal appearance weight and a low dot gain value is intended to represent lower tonal appearance weight. However, because dot gain is a value expressed as a measure relative to a specific solid density value, dot gain is always measured by first measuring a solid area, in close proximity to the 50% screened area, followed by measurement of that screened area. For example, a 50% dot area having an apparent dot area value of 72% is said to have a 22% dot gain.
Unfortunately, dot gain does not necessarily provide a reliable measurement in many applications. For example, a measured 22% dot gain for a 50% dot area may actually have a variety of screened area density values as compared to the solid density values that were measured. For example, solid density regions of 1.50, 1.30, and 1.10 may all actually yield the same 22% dot gain for screened area densities of 0.52, 0.50, and 0.47, respectively. These dot gain measurements may be obtained from the solid density measurements by a variety of methods, including using Murray-Davies equations. Thus, unfortunately, it is not easy to discern which of two or more dot gain values has the highest or lowest tonal appearance weight when the solid densities related to the 50% screened areas have solid density values that differ from each other.
Dot gain measurement data also falls short as a method to mathematically calculate differences between device reproduction characteristics, because it is highly unlikely that both processes will have similar solid density values for a given measurement. Subsequently, because dot gain does not provide an absolute measured value, it does not provide a good basis for use in calculating precise transformation factors to be performed on individual color channels without considering interaction between the color channels (one-dimensional transformation factors).
Most current press operations provide one-dimensional control (where colorants do not overlap when printed on a substrate such as paper) by using SWOP-certified proofing systems with the proper solid density requirements and the specified dot-gain-at-50% values when properly exposed. Typical press operational control of one-dimensional characteristics is achieved by proper selection, and controlled use, of elements such as paper, inks, plates, fountain solutions, image transferring cylinder blankets, press mechanical settings and ambient moisture/temperature conditions, among others. In addition, CTP technology may be utilized to gain more precise control of the tonal scale of each of the C, M, Y, K colorants. For example, in the process of making plates by computer controlled laser exposure, image data may be transformed as each plate is made to make every plate's image tone reproduction precisely fit the need of the particular press on which it will be used.
Unfortunately, in many cases results produced even after managing these press operations are often unacceptable. These inaccurate results may be caused by, among other things, an inability to precisely control solid density and dot-gain-at-50% on presses which are not always capable of meeting SWOP specifications. These inaccurate results may also appear when, even after adjustments have been made to achieve “proper” solid density requirements and specified dot-gain-at-50% values, other screened areas, such as the 5%, 10%, 25%, 75%, and 90%, still do not correspond to prepress proofing values. Moreover, the process of obtaining accurate results increases in complexity across production print jobs, because the subject matter printed on the press, especially customer-designated ‘crucial colors,’ changes with each production print job. Acceptance of each production print job usually involves a customer's subjective assessment as to whether these crucial colors printed on the press correspond to prepress proofing values, rather than any measurable or objective assessment.
Furthermore, many fluctuations in press printing conditions' printing characteristics including, but not limited to, variations due to paper/base substrates, inks, plates, fountain solutions, image transferring cylinder blankets, press mechanical settings and ambient moisture/temperature conditions may change batch-to-batch or day-to-day. These fluctuations usually affect the printing device's reproduction characteristics during each production print job. Unfortunately, it is not practical to track down these causes of day-to-day or batch-to-batch variations and correct them before running a production print job. The traditional approach to accommodate these variations is to adjust ink film thickness, which usually accommodates one area at the expense of others. The print buyer is thus usually forced to compromise quality. Traditional press check procedures, which include a press operator's subjective color adjusting to meet a customer's needs, also offer no objective feedback to aid the decision-making process prior to doing the adjusting.
In addition, traditional make-ready procedures are often burdensome and waste precious time and resources. For example, these procedures usually include tasks that are done iteratively for each press sheet randomly selected for evaluation until the procedure achieves settings required for that production run. These tasks usually include using a color bar with color samples distributed without any defined spatial relationship to either a particular reference point or to ink fountain zone controls, taking measurements by a handheld device, and manually annotating, directly on the press sheet being evaluated, density readings in close proximity to color samples. These tasks also include informal selection of target solid density aimpoints and tolerances for variation, usually by the press operator. Then a determination is usually made as to whether, and to what degree, any adjustments are required.
Usually the densities that have provided the most recent best results are used as the chosen targets. In addition, if the adjustment on the press is being done by remote control at the press console, the press operator aligns the press sheet with the scale on the press console representing the array of ink fountain zone controls, and visually translates color sample positions into ink fountain zone control positions. The operator then uses his own subjective experience to translate these annotations into ink control settings and makes adjustments by executing commands on the console's remote controls (such as by pushing buttons and observing the console's display). On the other hand, if the adjustment on the press is being done directly on the ink fountain by manually operating the mechanisms, the press operator carries the annotated press sheet to the vicinity of each ink fountain of each printing unit, aligns the press sheet to the ink fountain zone controls, visually translates color sample positions into ink fountain zone control positions, similarly translates these annotations into ink control settings, and makes the adjustments by exerting force to the mechanisms (such as by turning screws). Unfortunately, these efforts to achieve the targeted solid density aimpoints during the press make-ready phase are usually abandoned early in the process and replaced during the press check phase with the goal of simply making the color of a printed sheet look like the color on a proof by regulating the ink film thickness in selected areas across the sheet. This process is both burdensome and wastes time and materials.
There have been some recently developed methods of performing make-ready procedures, including those described in U.S. Pat. Nos. 4,881,181, and 4,947,746. Unfortunately, these methods typically require detailed setup by operators using methods that relate to a particular printing press or press model and a particular color bar that may be used for the particular printing press or press model. These systems also typically require entries for the quantity of ink fountain zone controls and the positions for each of the centerpoints of these ink fountain zone controls, which may approximate 30 entries on a 40 inch press. These systems may also typically require entries for the position of each of the color measurement samples, which may approximate 30 per color, or 120 entries on a four-color 40 inch press. In addition, these methods require distance measurements of the color samples' relation to an exact reference point such as the center of a printing press. As a result, these methods may consume valuable resources involved in providing adjustments to ink fountain zone controls. Such methods require a great deal of time and may also be subject to errors resulting from these setup procedures.